My advice: don't attack Spits head on. They have extremely powerful deflector shields protecting the aircraft during a merge that are all but impervious to weapons fire, especially around the engine. This shield also appears to greatly reduce the durability of your own aircraft, as a single ping of .50 cal off a Spixteen is sufficient to knock out any other aircraft's engine in the merge.
And no, I'm NOT being sarcastic. I've unloaded a good 100+ rounds of MaDeuce right into the engine of a Spixteen during a front-quarter pass (kind of hard not to, because those things can generate HOs at will) and watched him fly away without even smoking. Whereas I've taken one hit during the exchange and find myself gliding.